1. Field of the Invention
The current invention relates to a method and device for suppressing the effect of signals that are received or sent via side lobes of an antenna of an amplitude or phase monopulse radar device.
2. Discussion of Background Information
In order to locate and if need be, track flying objects, normally monopulse radar devices are used, as have been described, among others, in S. M. Sherman, "Monopulse Princeples and Techniques", Artech House, Norwood 1984. With monopulse radar devices, angular error signals in azimuth and elevation are generated with each received pulse, which become zero when the antenna axis or bore sight axis is aimed precisely at the target. The orientation and if need be tracking of the bore sight axis is executed mechanically in horn antennae. Often in array antennae, the measurement range in elevation is scanned electronically and the measurement range in azimuth is scanned mechanically (see A. E. Acker, "How to Speak Radar", Basic Fundamentals and Applications of Radar, Varian Associates, Palo Alto, 1988, pp. 30 & 31).
Known amplitude or phase monopulse radar devices supply precise angular measurement data with regard to a flying object, provided that in addition to the signals that are received directly from the monitored flying object, no signals from other objects and no singly or multiply reflected signals from the first or the other objects are received.
FIG. 1 shows two flying objects T1 and T2 that have been detected by the main lobe ml of an antenna, of which the first is disposed above the bore sight axis bx and the second is disposed below this axis (see U.S. Pat. No. 4,672,378, FIG. 6). According to S. M. Sherman, chapter 8, pp. 201-210, the spatial position of a flying object can no longer be precisely determined with a conventional monopulse radar device as soon as a second flying object is disposed in the same radar beam. In comparison to the single-target case, the phase of the resulting differential signal changes in relation to the sum signal. In addition, conventional monopulse radar devices supply erroneous angular measurement data as long as the targets cannot be separated distance-wise. A special instance of the two-target case is when the radar echo reflected by a flying object is reflected, for example, on the surface of water. Also depicted is the fact that a third flying object T3 has been detected by a side lobe sl of the antenna, which impairs the measurement of the first two flying objects T1 and T2 or, (in the absence of objects T1, T2) incorrectly indicates a detection of the object T3 in the direction of the main lobe.
Erroneous measurements of objects flying over water are prevented by means of the process known from Dr. A. Schenkel, "Crossfeed Monopulse--a Specific Method to Eliminate Mistracking Over Sea", presented at the International Conference "Radar 87", London, Oct. 19-21, 1987. Although the (crossfeed) process described has brought fundamental improvements over the conventional monopulse process, under certain circumstances, measurement errors must also be reckoned with in this process. The shortcomings of the crossfeed process were eliminated by the curvature process known from WO 97/22890, in which three antenna functions (sum, difference, curvature) are used for each measurement axis.
Neither the classic monopulse process, the crossfeed process, nor the curvature process permit a determination to be made as to whether single targets or double targets have been received from the direction of the main lobe ml or via side lobes sl of the antenna. In addition, interference signals are often received from the side lobe region sl, which impair the processing of the signals of the actual targets. Signals from the side lobe region therefore should preferably be blanked.
So-called SLS and SLC processes are used to suppress signals that are received from the side lobe region. Through the SLS process (side lobe blanking or side lobe suppression), a suitable measurement method is used to determine whether a signal has been received from the side lobe region. Depending on the measurement result, the received signals are either processed further or are completely suppressed. In the latter case, the useful part of the signal is also lost. In contrast, with the SLC process (side lobe cancellation), a corresponding shaping of the antenna beam achieves the fact that the antenna function has a zero point in the angular range in which the interference signal occurs. As a result, the interference signal is suppressed without impairing the useful signal.
With the SLS process, which is described by way of example in M. Skolnik, "Radar Handbook", McGraw Hill, New York 1970, p. 29-18 or R. C. Johnson, "Antenna Engineering Handbook", 3rd edition, McGraw Hill Book Company, New York, 1993, p. 33-6, in addition to the main antenna, an omni-directional auxiliary antenna is used, which has a virtually constant, but low antenna gain over a large angular range and preferably has a minimum in the direction of the bore sight axis of the main antenna. The signals of both antennae are supplied by way of separate reception stages to a comparator, which produces the quotients of the signal power received by the auxiliary antenna and the main antenna. The effect of the received signal, namely its depiction on the screen as a radar echo, is suppressed so long as the quotient exceeds a predetermined threshold value (RSLS process). Through a suitable adjustment of the threshold value, the angular range can be determined within which the signals are associated with the side lobe and are consequently suppressed. In the ISLS process, the effect, namely the transmission of a transponder response, is suppressed when the transponder is queried by way of the side lobe of a base station.
According to R. C. Johnson, auxiliary antennae whose phase centers do not coincide with that of the main antenna are only partially suitable for RSLS and ISLS processes.
In the curvature process known from WO 97/22890, for the additional execution of the SLS process, one of the antenna functions, the curvature function, can be additionally used as an auxiliary function. Due to the requirement for a particular progression of the curvature function, however, limitations arise that counteract an optimization of the curvature process.
In addition to the three antenna functions provided for target measurement (sum, difference, and curvature), curvature monopulse radar devices have up to now required another auxiliary function in order to use the SLS process according to U.S. Pat. No. 4,450,448, which resulted in a higher cost. In U.S. Pat. No. 4,672,378, conventional monopulse radar devices are described, for which an auxiliary function is embodied for the main antenna in order to carry out the beam production in the SLC process. To that end, illumination functions are provided, by means of which the weighting of the elementary signals of an array antenna are determined as a function of the location lx on the array. The transition from the illumination function to the corresponding antenna function is carried out by means of the Fourier transformation. Performing auxiliary functions on the main antenna permits the use of auxiliary antennae to be eliminated. Furthermore, the phase centers of the main and auxiliary functions are disposed close to one another, which produces various advantages described in U.S. Pat. No. 4,672,378. In contrast to the SLS process described, for example, in M. Skolnik or R. C. Johnson, the auxiliary antenna functions do not have an omni-directional characteristic curve, but have a high antenna gain in the incident direction of an interference signal. The received and weighted interference signals of the auxiliary paths are subtracted in an addition stage from the signal of the main path, whereupon the latter is largely freed of interference elements. The SLC process described in U.S. Pat. No. 4,672,378 required a number of antenna functions to suppress interference signals.